The present invention relates to pH electrodes, and is directed more particularly to pH electrodes which can withstand exposure to sterilizing temperatures and to a method for producing such electrodes.
Glass pH electrodes have long been known and used to measure the hydrogen ion activity of test solutions, particularly aqueous test solutions. Such electrodes typically include an electrode body which comprises a glass pH sensitive member that is attached to one end of an electrochemically inert support member, such as a tubular piece of glass or plastic. The interior of this electrode body is typically filled with a suitable electrolyte solution, such as a dilute buffered solution of aqueous potassium chloride. This electrolyte solution serves as an electrolytic bridge between the inner surface of the glass member and an internal reference such as a silver wire that is coated with silver chloride. In use, the potential difference across the glass member tends to vary, in accordance with the well known Nernst equation, at a rate of 59 milivolts per pH unit.
pH electrodes of the above described type operate satisfactorily so long as their temperatures remain between the freezing and boiling points of their electrolytes. At temperatures below the freezing point, pH electrodes tend to become damaged by the thermal expansion that is associated with the freezing of the water in their electrolytes. Similarly, at temperatures above the boiling point, pH electrodes tend to become damaged by the internal pressure that is associated with the vaporization of the water in their electrolytes. Thus, the use of aqueous electrolyte solutions limits the range of temperatures over which glass pH electrodes can be used.
While the above described temperature limitations are acceptable in many applications, they are not acceptable in applications in which an electrode is used to measure the pH of a liquid food product or the contents of a fermentation vat. This is because applications of the latter type require the maintenance of totally or selectively sterile conditions. The latter conditions, in turn, require that all of the equipment that comes into contact with the liquid, to be measured must first be sterilized. Usually, such sterilization requires the exposure of the equipment to temperatures of approximately 140.degree. C. for periods of several minutes. Moreover, such sterilizations must be repeated on a regular basis.
One solution to the sterilization problem has been to sterilize pH electrodes by nonthermal means, such as by exposure to ethylene oxide or other fluids having bactericidal properties. Sterilization procedures of this type, however, are undesirable because they require the removal and reinstallation of the electrode. This removal and reinstallation, in turn, is undesirable because it breaks the integrity of the liquid containment system, and thereby creates a risk that live bacteria will be introduced.
Another approach to the sterilization problem has been to use pH electrodes which have dry, solid-state electrolyte systems. Because such electrodes do not contain any liquid, they do not produce the internal pressures which damage the delicate glass pH sensing member. They also, however, show a reduced pH sensitivity and tend to produce unstable pH readings.